1. Field of the Invention
The present invention relates generally to surgical instruments, and more particularly to a motor drive unit for use in conjunction with an atherectomy device.
Atherectomy devices have been developed for the treatment of coronary artery disease by cutting diseased tissue along the wall of an artery. To accomplish this, an elongated catheter shaft having a cutting blade at its distal end is positioned in the artery near the diseased region. The cutting blade is attached to a drive cable which extends through the shaft from the proximal end, where it is coupled to a motor drive unit. The motor drive unit applies torque to the drive cable in order to rotate the cutting blade, thereby severing diseased tissue.
U.S. Pat. No. 4,771,774 to Simpson, et al., issued Sep. 20, 1988, the full disclosure of which is incorporated herein by reference, describes a motor drive unit for use in connection with an atherectomy device. The motor drive unit includes a case having a size and shape appropriate for being hand held. A motor is mounted in the case, along with a power supply, usually comprising one or more batteries, providing power to the motor. A switch is mounted in the case and is accessible for finger operation by the user. The motor drive unit connects to the proximal end of the shaft of the atherectomy device. Upon activation of the switch, the motor applies a torque to the proximal end of the drive cable, thereby rotating the cutting blade.
Although such atherectomy devices are generally effective, the motor drive unit used with such devices has suffered from certain problems. In particular, upon start-up, the motor drive unit has been found to apply a torque to the drive cable of the device of such magnitude that the cable fails. Typically, this occurs when the cutting blade and/or the drive cable are bound due to engagement of the blade or a tortuous configuration of the shaft. In such a situation, when the switch is turned on, the motor instantaneously applies a torque pulse to the drive cable which exceeds the torsional strength limit of the cable.
Therefore, an improved motor drive unit for use with an atherectomy device is desired which would limit the initial torque applied by the motor to the drive cable upon start-up to a level lower than the torsional strength limit of the cable. Preferably, the motor drive unit should undergo a ramp-up from a power-off state to a power level sufficient to turn the blade and cable, providing a smooth and continuous increase in power delivered from the power supply to the motor. Further, the performance of the motor drive unit should not be affected once the blade has begun to turn.